1. Field of the Invention
The present invention relates to active noise cancellation systems, and more particularly, to headsets utilizing active noise cancellation.
2. Description of the Related Art
Conventionally, passive headsets and over-the-ear earplugs comprise a pair of earpieces coupled by a resilient headband. An annular foam pad attached to each earpiece forms a cushion between the shell of the earpiece and the user's head. The resilient headband presses the earpieces against the user's head. Ambient sound is attenuated before it reaches the wearer's ear by occlusion of sound by the earpieces and absorption of transmitted sound by materials within the earpieces. The degree of attenuation achieved depends upon the nature of the ambient noise and the qualities and characteristics of the individual headset or earplugs.
In various applications, however, passive attenuation is insufficient. Some environments are simply too noisy for comfort or even safety with only passive earplugs. In other environments, the elimination of extraneous noise is paramount, and satisfactory results cannot be achieved using passive means. Although the amplitude of the extraneous noise may be significantly diminished, it is almost impossible to completely isolate the wearer from extraneous noise using passive means. In addition, passive earplugs attenuate all sound, regardless of whether the wearer needs or wants to hear particular sounds.
Active noise cancellation systems eliminate unwanted sound using destructive interference. Cancellation is achieved by propagating anti-noise, identical to the unwanted soundwaves but inverted, which interacts with the unwanted waveform and results in cancellation. A feedback active cancellation headset typically includes a sound generator in each earpiece for producing anti-noise, and a residual microphone, also located in each earpiece, to provide feedback signals to a controller which generates the proper anti-noise signals. Each microphone detects the unwanted noise within each earpiece and provides corresponding signals to the controller. The controller supplies anti-noise signals to the sound generator corresponding to the noise detected in the earpieces, but inverted, with respect to the unwanted waveform. When the anti-noise interacts with the noise within each earpiece, destructive interference between the noise and the anti-noise cancels the unwanted sound.
Ideally, the residual microphone in feedback systems perceives the same sounds as the eardrum of the listener. In this regard, effective proximity to the eardrum is vital; the goal of the cancellation systems is to reduce the unwanted noise at the eardrum to zero, but in fact operates upon the noise detected by the microphone. Consequently, it is desirable that the microphone be placed sufficiently close to the eardrum to detect a reasonably similar noise field to that perceived by the listener. The eardrum, however, is located deep within the ear canal. Placing a microphone within the ear canal is generally impractical and very uncomfortable for the user. In addition, locating the microphone a significant distance away from the sound generator introduces a phase shift between the residual and cancellation signals and causes instability. As a result, active cancellation systems conventionally approximate the sound perceived by the listener by locating the microphone as close to the ear canal as possible without actually penetrating it.
Conventionally, the microphone is placed directly between the sound generator and the ear in axial alignment with the sound generator, and hence the anti-noise field, e.g. disposed at the center of a grille covering the sound generator. For headsets that do not form an acoustic seal between the earpiece and the user's head, the cancellation sound detected such a center-disposed microphone is significantly different from the cancellation sound perceived by the user. Because of the proximity of the microphone to the sound generator and center of the anti-noise field, the cancellation sound detected by the microphone is attenuated very little. As the point of measurement moves away from the sound generator, the level of detected sound decreases due to the relatively low acoustic resistance of the foam cushion between the earpiece shell and the user's head. Thus, the cancellation sound incident upon the user's eardrum is of a significantly lower amplitude than the primary sound detected by the microphone. Consequently, the effectiveness of the cancellation system suffers because the anti-noise in the ear canal is insufficient to cancel the soundwaves.
To enhance the low frequency response, some headphones use an open back design. For example, in many designs, a series of perforations are formed in the back of the headset, allowing air to move in and out of the chamber behind the sound generator to enhance lower frequency response. While the headphones are in use, however, the perforations may be covered or obstructed, for example by the user's hands, or by a pillow on an airplane. When the perforations are obstructed, the low frequency sensitivity of the headphones drops. In addition, obstruction of the perforations changes the frequency response of feedforward systems in midband frequencies that are within crucial cancellation frequency ranges.
Another problem associated with feedback cancellation systems is that they are prone to instability. Feedback systems tend to become unstable, for example, if the bandwidth of the system is too broad or the gain of the system is too high. When instability occurs, the system usually emits a loud noise that is generally unpleasant and occasionally dangerous. Consequently, the maximum range and effectiveness of feedback systems are limited by parameters designed to keep the feedback system stable.
To effect maximum cancellation, the waveform of the interacting anti-noise should exactly match the unwanted waveform, but should be inverted. The acoustic properties of each earpiece, however, affect the characteristics of the anti-noise waveform. The effect of the acoustic properties may be corrected by processing the residual signal according to a transfer function characteristic of the acoustic properties of the system to compensate for the effects. However, these acoustic properties of the headset are not constant under all conditions, and may vary with the force applied to the earpiece onto the user's head. When high pressure is applied to the earpiece, or when the headset is removed from the user's head, the variation of the earpiece's acoustic properties, particularly the volume and acoustic resistance, may cause instability in the feedback loop. This instability, in turn, causes the control loop to generate unstable oscillations, producing unpleasant and potentially even harmful noise.
In contrast to feedback systems, some active noise cancellation systems use feedforward techniques. In a feedforward system, an external microphone is placed in a noise field between the listener and the noise source. As the soundwaves propagate toward the listener, the microphone detects the noise before it reaches the listener's ears. The microphone provides a signal to a controller that generates a cancellation signal according to the detected noise. The cancellation signal is provided to a sound generator which generates anti-noise near the listener's ear to effect cancellation. Because the propagation speed of the soundwaves and the distance between the listener's ear and the external microphone are known, the controller precisely times the generation of the cancellation signal so that the proper phase relationship with the undesired sound is maintained and adjusts the amplitude to provide the correct cancellation level. Like feedback systems, application of abnormal force to an earpiece using a feedforward cancellation system disrupts the operation of the active noise cancellation system. Instead of causing instability, however, changes in the acoustic properties reduce the effectiveness of the cancellation system.
In at least one system, as disclosed in PCT Application PCT/US91/06636, filed Sep. 13, 1991, by Todter, et al., a feedforward system is purported to be combined with a feedback system to provide added cancellation. The feedforward signal is mixed with the cancellation signal generated by the cancellation circuitry and applied as a drive signal to the sound generator. However, the feedforward signal, when mixed with the cancellation signal to drive the sound generator, is in conflict with the feedback operation of the circuit, i.e. the feedforward signal is itself subject to cancellation. Consequently, although the feedforward system may add to the cancellation to some extent, its effectiveness is reduced due to cancellation of the feedforward signal by the feedback system.
The voltage level supplied to the system's power amplifier tends to affect the level of amplification, but also affects power consumption. In many applications, active noise canceling headsets employ batteries to supply power to the controller, the sound generating unit, and microphones. Use of batteries avoids the necessity of a power cable connecting the headset to a power source. A power cable tends to reduce the mobility of the headset wearer and presents a hazard to others in the area. Batteries, on the other hand, have only a limited operational lifespan before needing replacement or recharging. Frequent replacement or recharging of the batteries costs money and time, and imposes an inconvenience on the user.
In addition, many noise cancellation headsets are designed not only to cancel unwanted noise, but to provide particular sounds to the user. For example, headphones for listening to music or for use by pilots ideally cancel extraneous noise, and transmit particular desired sounds to the listener. Conventionally, the desired input signal is mixed with the residual signal from the internal microphone so that the desired signal is not canceled by the system. Feedback noise cancellation systems, however, because of their limited bandwidth, exhibit a high frequency rolloff having a relatively low cutoff frequency. Because of this cutoff frequency, higher frequencies of the desired sound tend to be attenuated, degrading the quality of the signal. Consequently, an equalizer must be added to return the sound to its proper amplitude.